The present invention relates to a magnetic mirror plasma source. Before turning to the detailed description of the presently preferred embodiments, related prior art is discussed below. The related prior art is grouped into the following sections: magnetic confinement and the Penning cell source, facing target sputtering, plasma treatment with a web on a drum, and other prior art methods and apparatuses.
Magnetic Confinement and the Penning Cell Source
A good presentation on magnetic mirror confinement is presented in section 3.4.2 of J. Reece Roth, Industrial Plasma Engineering, Volume 1: Principles, IOP Publishing, Ltd. 1995. Confinement of electrons and ions using magnetic mirrors is well known, especially in fusion research.
Facing Target Sputtering
U.S. Pat. No. 4,963,524 to Yamazaki shows a method of producing superconducting material. An opposed target arrangement is used with the substrate positioned between the electrodes in the magnetic field. The magnetic field is symmetrical between the electrodes, and the substrate is in the middle of the gap. With the substrate in this position, the Hall current generated within the magnetic field tends to be distorted and broken. When this happens, the plasma is extinguished and/or the voltage is much higher.
Plasma Treatment with a Web on a Drum
In U.S. Pat. Nos. 5,224,441 and 5,364,665 to Felts et al., a flexible substrate is disposed around an electrified drum with magnetic field means opposite the drum behind grounded shielding. In this arrangement, the shield opposite the drum is either grounded or floating. The substrate is supported by the surface without a mirror magnetic field emanating from the substrate.
In U.S. Pat. No. 4,863,756 to Hartig et al., the substrate is continuously moved over a sputter magnetron surface with the surface facing the magnetron located inside the dark space region of the cathode. In this way, the magnetic field of the magnetron passes through the substrate and is closed over the substrate surface constricting the plasma onto the surface.
Other Prior Art Methods and Apparatuses
U.S. Pat. No. 5,627,435 to Jansen et al. discloses a hollow cathode source operating at high, diode plasma regime pressures (0.1-5 Torr). The plasma is created inside the housing and then is emitted through holes. The plasma is generated in one chamber and then conducted to the substrate with the help of magnets under the substrate.
U.S. Pat. Nos. 6,066,826 and 6,287,687 to Yializis and Yializis et al. disclose a plasma treatment device for web materials. Similar to Jansen et al., a plasma is generated with a hollow cathode array and is ‘focused’ on the web by a magnetic field. As stated in the patent, the web charges up and the treatment stops when DC is used. As will be made clear below, this is different than the presently preferred embodiments. In the presently preferred embodiments, DC can readily be used with an insulating substrate without charge buildup over time. In these referenced patents, the magnetic field lines are not shown.
U.S. Pat. No. 6,077,403 to Kobayashi et al. shows a magnetron in combination with a second magnetic field. In this patent, the second field passes through the substrate to a supplemental electrode. This apparatus is not a stand-alone plasma source—it assists with ionizing and directing sputtered material to the substrate. Also, the first embodiment has the mirror field with a stronger magnetic field at the supplemental electrode than at the surface of the substrate.
In U.S. Pat. No. 4,631,106 to Nakazato et al., magnets are located under a wafer to create a magnetron type field parallel to the wafer. The magnets are moved to even out the process. The opposed plate is grounded, and the wafer platen is electrified.
U.S. Pat. No. 4,761,219 to Sasaki et al. shows a magnetic field passing through a gap with the wafer on one electrode surface. In this case, the electrodes are opposed to each other. The wafer is placed on the less compressed magnetic mirror surface, and the opposed surface across from the wafer is grounded.
U.S. Pat. No. 4,853,102 to Tateishi et al. uses a cusp field to assist sputter deposition into high aspect ratio holes. The flux lines leaving the substrate do not enter a negatively biased electrode.
U.S. Pat. No. 5,099,790 to Kawakami shows a microwave source with a moving magnet below the wafer to even out the coating on the wafer. In another figure, the substrates are moved over a stationary magnet(s). In this source, the plasma is generated in a separate plasma generation chamber and then directed to the wafer substrate with the assistance of the magnet under the substrate.
In U.S. Pat. No. 5,225,024 to Hanley et al., ExB containment is achieved by forcing the B flux into a parallel path over the substrate surface. U.S. Pat. No. 5,437,725 to Schuster et al. discloses a metal web drawn over a drum containing magnets. The web is electrified, and the opposed shield is at ground potential.
The source disclosed in U.S. Pat. No. 5,900,284 to Hu produces several magnetron type confinement traps on the surface above the magnets.